The invention relates to a photosensor and, more particularly, to a photosensor suited for an image sensor.
A photodiode using hydrogenated amorphous silicon (a-Si:H) film has attracting attention as an image sensor element for use in such an apparatus as an electronic copier and a facsimile apparatus. Since the a-Si:H film has high photosensitivity and can be easily made large in area, it is particularly suited for a contact type image sensor.
An image sensor is formed by arranging a great number of photodiodes in line. The photodiode usually takes a sandwich structure in which an a-Si:H film is sandwiched between upper and lower electrodes. The lower electrode is made of chromium (Cr) or like, while the upper electrode is made of transparent, electrically conductive film such as indium tin oxide (ITO) film. In this structure, a Schottky barrier is formed at interface between the a-Si:H film and upper electrode, thus exhibiting a diode characteristic. The lower electrode serves as a cathode electrode, and the upper electrode as an anode electrode.
A large number of such photodiodes are formed in line on a substrate, thus constituting an image sensor. The anode of each photodiode is connected to the negative terminal of a DC power supply, and the cathode of the photodiode is connected through a charging switch to the positive terminal of the DC power supply. The cathode of the photodiode is connected through a charge readout switch to a common output amplifier. When the charging switch is turned on, inter-electrode capacitor of the photodiode is charged. When the charging switch is turned off, the inter-electrode capacitor is discharged in response to light incident on the anode electrode. Subsequently, the charge readout switch is turned on, so that a signal voltage across the capacitor is read out by the output amplifier.
The signal voltage taken from the capacitor is proportional to the product of the light-receiving area, illumination intensity and storage time. The recent trend for higher operation speed and higher resolution calls for smaller light-receiving area and shorter storage time; for instance, a resolution of 16 bits/mm and a storage time of 1 msec. are required. The light-receiving area of one photodiode corresponding to the resolution of 16 bits/mm is 3.91.times.10.sup.-3 /mm.sup.2. In this case, the inter-electrode capacitance is approximately 0.2 pF. Under conditions of a practical illumination intensity of 500 lx and an internal quantum efficiency of 1, the maximum signal voltage is found to be approximately 6.8 V by calculation.
The noise in the image sensor includes a switching noize component resulting from the switch element (i.e., transistor) and a dark current noise component based on dark current in the photodiode. While the switching noise is irrelevant to temperature, the dark current noise increases with temperature. At 40.degree. C. or below, the signal-to-noise ratio of the read-out signal is 38 dB at the most, which is insufficient in practice. Recently, a signal-to-noise ratio of 80 dB or above in a temperature range of 0 to 80.degree. C. is required in addition to the high operation speed and high resolution.
As another prior art photosensor, a combination of a solar battery and a MOS transistor is known, as is disclosed in Japanese Patent early Publication No. 59-25280 published on Feb. 9, 1984. In this structure, a solar battery using a-Si:H film is formed on a MOS transistor formed on a silicon substrate, connected between the gate and source of this transistor. This drain current of the MOS transistor is controlled by incident light on the solar battery. The substrate is made of silicon crystal, and cannot easily be made large. Therefore, the structure is not suited for a photosensor for use in an image sensor which requires a large area.